1. Field of the Invention
This invention relates to a fuel injection controlling apparatus for reducing NOx amounts in an exhaust gas of a Diesel engine that executes lean combustion.
2. Description of the Related Art
Various proposals have been made in the past to reduce amounts of NOx contained in an exhaust gas of a Diesel engine. For instance, Japanese Patent Laid-Open Nos. 218920/1996 and 358717/1992 describe one of such proposals.
The proposal described in Japanese Patent Laid-Open No. 218920/1996 arranges a NOx absorbing agent that normally absorbs NOx, renders an air-fuel ratio of an exhaust gas rich, and emits and reduces NOx when the built-up amount exceeds a limit. When NOx is to be emitted from the NOx absorbing agent, an engine control circuit lowers an operation air excess ratio. At the same time, this control circuit advances fuel injection timing, increases a fuel amount to be injected into a combustion chamber before ignition from a fuel injection valve, and switches a combustion condition of the engine from a combustion condition mainly of normal diffusion combustion to a combustion condition mainly of pre-mixing combustion. In this way, this technology lowers the air excess ratio without generating smoke, renders the air-fuel ratio of the exhaust gas rich and conducts emission of NOx absorbed by the NOx absorbing agent and reduction-purification.
The technology described in Japanese Patent Laid-Open No. 358717/1992 arranges a catalyst converter for reducing NOx and a lean sensor inside an exhaust passage of a Diesel engine. There are also arranged an inter-cylinder injector for injecting a fuel into a cylinder and an intake system injector for injecting the fuel into an intake system. The inter-cylinder injector injects the fuel into the cylinder during the normal operation. When NOx is emitted from the catalyst converter and is reduced, the amount of the fuel to be injected from the intake system injector is calculated from an engine load and the NOx amount. The intake system injector injects the fuel into the intake system on the basis of the calculation result in addition to fuel injection from the inter-cylinder injector, renders the air-fuel ratio of the exhaust rich and supplies HC required by the catalyst converter for reducing NOx.
However, the constructions described above involve the following problems.
In the construction described in Japanese Patent Laid-Open No. 218920/1996, the fuel amount injected into the combustion chamber from the fuel injection valve before ignition is increased by advancing the fuel injection timing so as to switch the engine combustion from the combustion mainly of diffusion combustion to the combustion mainly of pre-mixing combustion. In other words, the fuel is injected under the state where a piston position is low. In consequence, large amounts of the fuel directly adhere to the inner wall of a cylinder liner and are carbonized to thereby increase soot in oil. Since large amounts of the fuel are injected into the cylinder within a short time, mixing of air and the fuel does not easily become uniform, and fuel consumption gets deteriorated.
In the construction described in Japanese Patent Laid-Open No. 358717/1992, the inter-cylinder injector injects the fuel during the engine operation and diffusion combustion is made. When the catalyst converter requires HC for reducing NOx, the intake system injector further injects the fuel to the intake system. Therefore, large amounts of NOx are emitted even in a low engine load zone, and the scale of the catalyst converter must be increased. In addition, the amount of the fuel injected to the intake system increases and fuel consumption gets deteriorated.